The present invention relates to a voltage controlled oscillator circuit and a method for configuring a voltage controlled oscillator circuit.
In a voltage controlled oscillator (VCO) circuit an output signal can be generated with a frequency that varies in response to an input control voltage. VCOs can be utilized in many circuit applications wherein many of them demand a wide output frequency range for the VCO. One example of a circuit application of a VCO is a phase-locked loop (PLL) which is an important circuit block in data communication systems.
If a wide output frequency range of the VCO is desired, the VCO typically requires a large gain. The VCO gain, however, has the undesirable effect of amplifying any noise in the input voltage signal to the VCO, resulting in output signal jitter and adversely affects the overall performance of the phase-locked loop. It is thus in general advantageous to keep the VCO gain low.
In LC-tank based VCOs usually the oscillation frequency is tuned with voltage dependent capacitors, in particular NMOS or PMOS varactors. To achieve a low VCO gain and still a large tuning range of the VCO, the total tuning range can be divided into several possibly overlapping sub-ranges by using varactors for continued fine tuning and an arrangement of switched varactors or capacitors for coarse frequency selection.
In such a configuration, however, the VCO gain is different for different tuning curves. The reason is that with an increasing number of coarse tuning varactors switched on, the VCO gain gets lower since the relative capacity of the fine tuning varactors compared to the total tank capacity decreases. If such a VCO is utilized in a phase-locked loop, the change of the VCO gain would result in an unwanted change of the PLL bandwidth.